The present invention relates to a structure for enhancing cooling of generator rotors through surface profiling of the copper end turns and/or spaceblocks.
The power output rating of dynamoelectric machines, such as large turbo-generators, is often limited by the ability to provide additional current through the rotor field winding because of temperature limitations imposed on the electrical conductor insulation. Therefore, effective cooling of the rotor winding contributes directly to the output capability of the machine. This is especially true of the rotor end region, where direct, forced cooling is difficult and expensive due to the typical construction of these machines. As prevailing market trends require higher efficiency and higher reliability in lower cost, higher-power density generators, cooling the rotor end region becomes a limiting factor.
Turbo-generator rotors typically consist of concentric rectangular coils mounted in slots in a rotor. The end portions of the coils (commonly referred to as endwindings), which are beyond the support of the main rotor body, are typically supported against rotational forces by a retaining ring (see FIG. 1). Support blocks are placed intermittently between the concentric coil endwindings to maintain relative position and to add mechanical stability for axial loads, such as thermal loads (see FIG. 2). Additionally, the copper coils are constrained radially by the retaining ring on their outer radius, which counteracts centrifugal forces. The presence of the spaceblocks and retaining ring results in a number of coolant regions exposed to the copper coils. The primary coolant path is axial between the spindle and the bottom of the endwindings. Also, discrete cavities are formed between coils by the bounding surfaces of the coils, blocks and the inner surface of the retaining ring structure. The endwindings are exposed to coolant that is driven by rotational forces from radially below the endwindings into these cavities (see FIG. 3). This heat transfer tends to be low. This is because according to computed flow pathlines in a single rotating end winding cavity from a computational fluid dynamic analysis, the coolant flow enters the cavity, traverses through a primary circulation and exits the cavity. Typically, the circulation results in low heat transfer coefficients especially near the center of the cavity. Thus, while this is a means for heat removal in the endwindings, it is relatively inefficient.
Various schemes have been used to route additional cooling gas through the rotor end region. All of these cooling schemes rely on either (1) making cooling passages directly in the copper conductors by machining grooves or forming channels in the conductors, and then pumping the gas to some other region of the machine, and/or (2) creating regions of relatively higher and lower pressures with the addition of baffles, flow channels and pumping elements to force the cooling gas to pass over the conductor surfaces.
Some systems penetrate the highly stressed rotor retaining ring with radial holes to allow cooling gas to be pumped directly alongside the rotor endwindings and discharged into the air gap, although such systems can have only limited usefulness due to the high mechanical stress and fatigue life considerations relating to the retaining ring.
If the conventional forced rotor end cooling schemes are used, considerable complexity and cost are added to rotor construction. For example, directly cooled conductors must be machined or fabricated to form the cooling passages. In addition, an exit manifold must be provided to discharge the gas somewhere in the rotor. The forced cooling schemes require the rotor end region to be divided into separate pressure zones, with the addition of numerous baffles, flow channels and pumping elements which again add complexity and cost.
If none of these forced or direct cooling schemes are used, then the rotor endwindings are cooled passively. Passive cooling relies on the centrifugal and rotational forces of the rotor to circulate gas in the blind, deadend cavities formed between concentric rotor windings. Passive cooling of rotor endwindings is sometimes also called xe2x80x9cfree convectionxe2x80x9d cooling.
Passive cooling provides the advantage of minimum complexity and cost, although heat removal capability is diminished when compared with the active systems of direct and forced cooling. Any cooling gas entering the cavities between concentric rotor windings must exit through the same opening since these cavities are otherwise enclosedxe2x80x94the four xe2x80x9cside wallsxe2x80x9d of a typical cavity are formed by the concentric conductors and the insulating blocks that separate them, with the xe2x80x9cbottomxe2x80x9d (radially outward) wall formed by the retaining ring that supports the endwindings against rotation. Cooling gas enters from the annular space between the conductors and the rotor spindle. Heat removal is thus limited by the low circulation velocity of the gas in the cavity and the limited amount of the gas that can enter and leave these spaces.
In typical configurations, the cooling gas in the end region has not yet been fully accelerated to rotor speed, that is, the cooling gas is rotating at part rotor speed. As the fluid is driven into a cavity by means of the relative velocity impact between the rotor and the fluid, the heat transfer coefficient is typically highest near the spaceblock that is downstream relative to the flow directionxe2x80x94where the fluid enters with high momentum and where the fluid coolant is coldest. The heat transfer coefficient is also typically high around the cavity periphery. The center of the cavity receives the least cooling.
Increasing the heat removal capability of passive cooling systems will increase the current carrying capability of the rotor providing increased rating capability of the generator whole maintaining the advantage of low cost, simple and reliable construction.
U.S. Pat. No. 5,644,179, the disclosure of which is incorporated by reference, describes a method for augmenting heat transfer by increasing the flow velocity of the large single flow circulation cell by introducing additional cooling flow directly into, and in the same direction as, the naturally occurring flow cell. While this method increases the heat transfer in the cavity by augmenting the strength of the circulation cell, the center region of the rotor cavity was still left with low velocity and therefore low heat transfer. The same low heat transfer still persists in the corner regions.
The invention enhances the heat transfer rate from the copper end turns of the field endwinding region by using surface machining or preparation to generate flow structures beneficial to cooling of the end turns. Improving cooling of the end turns in this region will provide the opportunity to increase the power output rating of a given machine leading to an improved cost basis on a dollar per kilowatt-hour basis. As the endwinding region is usually limiting in terms of satisfying maximum temperature constraints, improvements in this region should produce significant performance benefits.
Heat transfer rates are increased by the increased surface area, improved turbulent mixing on the surface, and boundary layer breakup and subsequent reattachment. According to a first embodiment of the invention, at least one of the copper end turns is machined to increase the surface area thereof as compared to a planar surface. This may be accomplished by roughening the surface such as for example, by creating grooves.
According to a second, alternate embodiment of the invention, the surface area of the end turns is increased by forming small dimples, similar to those provided on the surface of golf balls, on the rotor copper end turn sections. These dimples increase heat transfer rates by a factor of three or four while causing a negligible increase in the friction characteristics and overall pressure loss.
In accordance with a further feature of the invention, in addition to or rather than modifying the copper end turns themselves, the support blocks or spaceblocks disposed between the copper end turns are modified. According to one exemplary embodiment, turbulators are placed on the spaceblock face disposed on the downstream side of the cavity. More specifically, each of the rotor spaceblocks is fabricated with roughness elements. These turbulators act to disturb the flow, leading to increased turbulence and incoherent mixing. The result is to improve the overall heat transfer rate.
In another alternate embodiment, vortex generators are formed on the spaceblock face on the downstream side of the cavity. More specifically, for example, triangular sections are fabricated onto the spaceblock for the purpose of generating coherent vortex structures from the cooling gas flow across the spaceblocks.